1. Field of the Invention
The present invention relates to an optical address type display device which may be used as a display in the field of AV equipment such as a TV or a video game or OA equipment such as a personal computer or a wordprocessor.
2. Description of the Related Art
In recent years, a matrix type liquid crystal display device (LCD) has been requested to make its capacitance larger and larger. That is, with increase of resolution of a display device, the number of pixels has been requested to increase from 400.times.600 to 1000.times.1000 or more. The size of the display screen has been also requested to increase from 10 to 20 inches or more. In an active-matrix driving type LCD, in particular, a thin film transistor (TFT) driving type LCD, however, there may be brought about a problem that the increase of scan lines leads to the increase of wire resistance and a delay of a signal waveform may be caused by the wire resistance and the floating capacitance or a problem that a voltage ratio of selected pixels to non-selected pixels cannot be obtained if the number of scan lines is larger than a threshold value in the simple-matrix driving type LCD. To solve these problems, there has been proposed a liquid crystal display device with a high resolution which is capable of easily increasing pixel driving current through the effect of a light switching function. (Toyo Rayon, Ltd.: Japanese Patent Lying Open No. Hei1-173016, Casio Calculator, Ltd.: Japanese Patent Lying Open No. Hei1-224727, Matushita Electronic Industry, Ltd.: Japanese Patent Lying Open No, Hei2-89029, Seiko-Epson, Ltd.: Japanese Patent Lying Open No. Hei2-134617, Sharp, Ltd.,: Japanese Patent Lying Open No. Hei3-263647, etc.)
Later, the description will be oriented to a method for driving an active-matrix driving type liquid crystal display device which is one kind of an optical address type liquid crystal display device (optical scan type liquid crystal display device) as referring to the drawings.
FIGS. 24 and 25 show an optical address type active-matrix driving type liquid crystal display device as disclosed in the Japanese Patent Application No. Hei5-100246 filed by the applicant of the present application.
In this display device, a basic substrate 21 composing a display panel includes a plurality of light waveguides Y.sub.1, Y.sub.2, . . . , Y.sub.n arranged vertically on a glass substrate 21a. A clad layer 23 is formed on the glass substrate 21a in a manner to cover these light waveguides Y.sub.1, Y.sub.2, . . . , Y.sub.n. Signal wires X.sub.1, X.sub.2, . . . , X.sub.m are arranged horizontally in a manner to be crossed with the light waveguides Y.sub.1, Y.sub.2, . . . , Y.sub.n. A pixel electrode is formed in a manner to be substantially buried in each of the areas defined by the light waveguides Y.sub.1, Y.sub.2, . . . , Y.sub.n and the signal wires X.sub.1, X.sub.2, . . . , X.sub.m. Light switching elements 26, 26, . . . each made of a photoconductive film are provided vertically between an extended portion of the pixel electrode 25 and the signal wires X.sub.1, X.sub.2, . . . , X.sub.m. Inside of the glass substrate 21a, a light cut-off layer 28a is provided in a manner to correspond to each of the light switching elements 26, 26, . . . . This light cut-off layer 28a serves to prevent light (outer light) from the outer surface of the glass substrate 21a from being incident to the light switching element 26.
On the opposed surface of the opposed substrate 22, there is formed an opposed electrode 29 made of a transparent conductive film. On the opposed surface of the opposed electrode 29, a light cut-off layer 29b is provided at the location corresponding to the light switching element 26 and serves to prevent light (outer light) from the outer surface of the opposed substrate 22 from being incident to the light switching element 26. On the inside of each of the glass substrates 21a and 22a, an orientation film 27a or 27b is formed and is subject to an orientating treatment. The substrates 21 and 22 formed as described above are pasted with each other through a seal 32 and a display medium 33 laid therebetween.
In such an optical display type display device, when a ray of light is applied from a luminous element array 30 to each light switching element 26 through a micro lens array 31, light waveguides Y.sub.1, Y.sub.2, . . . , Y.sub.n, the light switching element 26 lowers its impedance so as to allow a signal voltage to be applied to the light switching element 26, thereby electrically connecting the signal wires X.sub.1, X.sub.2, . . . , X.sub.m with the pixel electrode 25. When no light is applied to the light switching element 26, the light switching element 28 enhances its impedance. This results in electrically insulating the signal wires X.sub.1, X.sub.2, . . . X.sub.m from the pixel electrode 25. That is, this optical address type display device is arranged to use a scan signal as a light signal and is driven by using the change of impedance of the light switching element 26.
FIGS. 26 and 27 show a positional relation among the light switching element 26, one light waveguide (for example, Y.sub.n), one signal wire (for example, X.sub.m) and a pixel electrode 25, on which the detailed explanation about it will be expanded.
This display apparatus uses a method for picking up light from light scattering portions 24, 24 . . . by forming flaws which are located on the part of the light waveguide Y.sub.1, Y.sub.2, . . . , Y.sub.n corresponding to the light switching elements 26 for supplying a ray of light from the light waveguide Y.sub.1, Y.sub.2, . . . Y.sub.n to light switching elements 26, . . . effectively.
The part of the ray of light propagating through the light waveguides Y.sub.1, Y.sub.2, . . . Y.sub.n are scattered at these light scattering portions and are emitted to the light switching elements 26 as a signal light.
In order to implement a high-density representation in the optical address type display device, it is necessary to provide a lot of light scattering portions 24 in one light waveguide. For example, in a high-definition TV (HDTV), 1000 or more signal wires X.sub.1, X.sub.2, . . . X.sub.m are required. 1000 to 5000 light switching elements 26 and light scattering portions 24 are required for one of the light waveguides Y.sub.1, Y.sub.2, . . . , Y.sub.n. However, the increase of the light switching elements 26 and the light scattering portions 24 in number brings about the following problem indicated below.
FIG. 28 shows a relation between quantity of light coming out of an end of the light waveguide and the number of the light scattering portions 24 on the light waveguide in the case that the light scattering portions 24 provided in all the light waveguides are the same with each other in size and form and a ray of light applies from the other end of the light waveguide. As will be understood from this figure, the quantity of light passed through the light waveguide and picked up out of the end of the light waveguide is attenuated exponentially as the light scattering portions are increased in number. The attenuation not proportional to the number but exponentially indicates that the quantity of picked light is different at each location. That is, the quantity of light picked up at each location is made smaller as the location goes further along the light waveguide from the light-incident side. In this prior art, since the light scattering portions 24 are the same in size, the signal light is attenuated while it is propagating through the light waveguide. As the light scattering portion 24 goes further from the light-incident side, the quantity of light picked up at the portion 24 is progressively made smaller. FIG. 29 shows a V groove of each light pick-up portion 24 and a light-applied state at the light pick-up portion 24 in the prior art.
In order to obtain even display performance on the screen of the display device, it is necessary to give even performance to all the light switching elements 26. For this purpose, the same quantity of light is required to be picked up at each light scattering portion 24. Hence, it is necessary to improve exponential attenuation of the quantity of picked-up light along the light waveguide.
As described above, such a display device as impairing a light waveguide for forming a V groove as means for picking up light or such a display device as making the surface of the light waveguide coarse as disclosed in the Japanese Patent Lying Open No. Hei 1-224727 is required to mechanically or chemically work the light waveguide. However, the mechanical work may often impair a glass substrate. To prevent the impair, a high-level working technique is required. As stated above, in the case of forming a lot of light scattering portions, the working accuracy may be insufficient. As the chemical work, a wet etching technique with a hydrogen fluoride etchant is used. In this case, the etching technique has difficulty in controlling the form and the size of the light pick-up portion. This results in making the reproducibility worse. To provide the light pick-up portion, a method for impairing the light waveguide or making the surface coarse may be provided. This method makes it impossible to apply 100% of the scattered light obtained from the light pick-up portion to the switching element, thereby making the light utilization efficiency worse.
FIG. 16 is a plan view showing a structure of an optical address type active-matrix driving type LCD. FIG. 17 is a section cut on the A--A line of FIG. 16. In the plan view shown in FIG. 16, a glass substrate 105b, a light cut-off layer 110, an orientation film 109b, a transparent electrode 106, a seal 107, and a liquid crystal layer 108 are not shown though they are shown on the section shown in FIG. 17.
As shown in FIGS. 16 and 17, on one glass substrate 105a, a plurality of linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n are ranged in the Y direction. On these linear luminous sources, a plurality of linear electrodes X.sub.1, X.sub.2, . . . , X.sub.m-1, X.sub.m are ranged in the X direction and in a manner to be crossed with the linear luminous sources, respectively.
Each of the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n, for example, the linear luminous source Y.sub.2, is composed of a luminous portion 101 formed of an LD or an LED array element and a linear waveguide 102 for transmitting a ray of light from the luminous portion 101, and a light pick-up portion 116 formed on the linear light waveguide 102. By operating the luminous portion 101, the light is propagated through the linear light waveguide 102 and is applied to the upper part of the substrate through the effect of the light pick-up portion 116.
At each of the crosspoints between the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y and the linear electrodes X.sub.1, X.sub.2, . . . , X.sub.m-1, X.sub.m, that is, on the light pick-up portion 116 of each of the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n, there is provided a light switching element 103 made of a photoconductive layer. The linear electrodes X.sub.1, X.sub.2, . . . , X.sub.m-1, X.sub.m are formed on the same side as the pixel electrode for driving a display medium, that is, liquid crystal. The light switching elements 103 are provided between the linear electrodes X.sub.1, X.sub.2, . . . , X.sub.m-1, X.sub.m and the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n, respectively.
On the other glass substrate 105b, a transparent electrode 106 is formed. The liquid crystal layer 108 is sealed between the substrate and the sealing member 107.
When a ray of light is applied to the light switching element 103, that is, the linear luminous source Y.sub.2 is made operative, the light switching element 103 lowers its impedance so that a signal from the linear electrode X.sub.1 may be applied to the pixel electrode 104 for changing an orientating state of liquid crystal.
By operating the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n sequentially from Y.sub.1 to Y.sub.n for optical scan, an electric signal may be correspondingly applied to the linear electrodes X.sub.1, X.sub.2, . . . , X.sub.m-1, X.sub.m. While the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n are made luminous, the light switching element on the linear luminous source is switched on. Hence, the electric signals from the linear electrodes X.sub.1, X.sub.2, . . . , X.sub.m-1, X.sub.m may be applied to the pixel electrodes 104, respectively. That is, in place of an electric gate signal of a TFT film, the light signals from the linear luminous sources Y.sub.1, Y.sub.2, . . . , Y.sub.n-1, Y.sub.n serve to scan the light switching element 103.
In the optical scan type liquid crystal display element, a technique for forming a light waveguide and a method for picking up light are important to controlling light for switching the liquid crystal. As the technique for forming a light waveguide, there has been proposed a method for melting an optical fiber on the glass substrate for forming a highly reliable and low-loss light waveguide (Sharp, Ltd.: Japanese Patent Application No. Hei4-4739)
As a method for picking up light, there have been proposed a method for picking up light scattered by a flaw on the light waveguide (Casio Calculator, Ltd.: Japanese Lying Open No. Hei1-224727, etc.).
However, a method for scattering light with a flaw has difficulty in controlling quantity of picked light when working the element. This is a disadvantage.